Flexible, porous ceramic composite film

ABSTRACT

A thin, flexible, porous ceramic composite (PCC) film useful as a separator for a molten-salt thermal battery comprises 50% to 95% by weight of electrically non-conductive ceramic fibers comprising a coating of magnesium oxide on the surface of the fibers in an amount in the range of 5% to 50% by weight. The ceramic fibers comprise Al 2 O 3 , AlSiO 2 , BN, AlN, or a mixture of two or more of the foregoing; and the magnesium oxide coating interconnects the ceramic fibers providing a porous network of magnesium oxide-coated fibers having a porosity of not less than 50% by volume. The pores of the film optionally can include a solid electrolyte salt. A laminated electrode/PCC film combination is also provided, as well as a thermal battery cell comprising the PCC film as a separator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Ser. No. 10/455,087,filed on Jun. 5, 2003, now abandoned, which claims the benefit ofprovisional application Ser. No. 60/386,859, filed on Jun. 6, 2002, thedisclosures of which are incorporated herein by reference.

STATEMENT OF GOVERNMENTAL RIGHTS

This invention was developed with Navy SBIR funding under Contract no.Contract N00167-99-C-0071. The U.S. government has certain rights inthis invention.

FIELD OF THE INVENTION

The present invention relates to porous ceramic composite films usefulas thin separators in thermal batteries. More particularly, thisinvention relates to flexible, porous ceramic composite films comprisingceramic fibers having a magnesium oxide coating, and to articles andbatteries including such thin films.

BACKGROUND OF THE INVENTION

Thermal batteries are the reserve power “of choice” on board many weaponand defense systems, due to their very long shelf life (about 25 years).The thermal batteries are kept in an essentially frozen state, untilactivated by heating. Within milliseconds of being heated to operatingtemperature, thermal batteries can produce very high pulse poweroutputs. Power generated by such batteries is utilized for guidance,communication, and arming of weapon and defense systems. Accordingly,thermal batteries play a critical role in our national defense.

Fiber mats of boron nitride (BN) suitable for use as separators inlithium-sulfide thermal batteries have been described by Hamilton, U.S.Pat. No. 4,284,610 and Maczuga, U.S. Pat. No. 4,354,986. The productionof BN fibers is described by Economy, U.S. Pat. No. 3,668,059. BNseparators exhibit structural stability (compressive strength), smallinterstices (useful as a particle barrier), and can hold a large volumefraction (65-85%) of a molten halide electrolyte. Because of itsunacceptable high production cost (e.g., due to difficult, hightemperature fabrication), and difficulty in initiating wetting withmolten halide, the use of BN as a separator has been abandoned in favorof high-surface area MgO powder-based separators. Pressed-powder MgOseparators are relatively inexpensive, have chemical stability, and canimmobilize 65-85 volume % of electrolyte within the interstices of thepressed powder. A significant drawback of MgO separators is the limitedstructural stability of the material, which results in undesirablelimits on thinness of the separator that can be obtained with thismaterial. A MgO powder separator when combined with molten electrolyte,is a paste at thermal cell operating temperatures.

Attempts to prepare MgO fibers have resulted in highly frangibleproducts, which are reduced to particles under compressive loads. U.S.Pat. No. 4,104,395 to Frankle teaches that impregnation of organicfibers with precursor materials can form mineral fibers after hightemperature processing (e.g., 1400° C.). Smith et al. (U.S. Pat. No.4,992,341) teach production of fiber-like sheets of MgO, by layering asheet of MgO powder in threads in a combustible binder, and thensintering the layered material to decompose the binder and form afiber-like MgO sheet structure.

Due its frangible nature, formation of a porous MgO structure withsufficient compressive strength for use in thermal batteries is limitedto materials having only about a 50% open volume fraction available forreceiving a working fluid, such as a molten halide electrolyte. Forexample, Briscoe et al. U.S. Pat. No. 5,714,283 describe formation ofMgO structure by sintering a MgO precursor (e.g., a soluble magnesiumsalt of an organic acid) on a microporous sintered metal screen support.The resulting sintered MgO films, having a thickness of about 3-25 mils,but have only about 20-50% open volume for incorporation of anelectrolyte. The low open volume of such materials imposes performancelimitations in thermal battery applications, especially if there isstructural disintegration (e.g., due to mechanical stressed duringmanufacture, etc.).

In thermal battery technology, the trend has been toward development ofhigher power density. The design approach for this has typicallyinvolved producing thinner cells. Thermal batteries are produced fromstacked cells consisting of pressed powder wafers in the followingrepeating order: (a) a heat pellet, (b) a Li-alloy negative electrode(anode), (c) a porous separator (e.g., MgO) containing a meltableelectrolyte salt, and (d) a FeS₂ positive electrode (cathode). Eachwafer typically is about 1 mm (about 39 mils) thick. Battery performancecould be improved by using a thinner separator, if suitable materialswere available. Thinner MgO separators are impractical due to thephysical strength limitations inherent in the MgO pressed-powdermaterials.

The separator component of a thermal battery physically separates andionically couples the anode and the cathode in each cell of the battery.Ideally, a separator should have a relatively high capacity for anelectrolyte and have connected porosity for high performance. Addedcharacteristics of importance include dimensional stability andflexibility. In one application, wafer thin separator components limitbattery pulse power to about 5.5 kW. A thinner separator would boost theproportion of active materials (electrolyte) in the battery, and thusboost power output. Unfortunately, MgO powder wafers have limitedhandling strength, and generally must be at least about 1 mm inthickness for practical use in thermal batteries. Thinner MgO tends tocrack or break, thus compromising the integrity of the entire battery.Larger diameter wafers exacerbate the handling problems. Because ofthis, MgO powder wafers must have a substantial thickness to be ofpractical use. In addition, volumetric changes of the active electrolytematerial tends to distort the electrolyte/separator interface, whichleads to cell shorting.

The prevailing construction and chemistry of a state-of-the-art thermalbattery has been around for about 25 years. It uses Li-alloy and metalsulfide electrodes, with a lithium halide salt as the electrolyte. Thesalt becomes molten upon heating. As noted above, the battery iscomposed of a stack of wafers of pelletized powders. Wafer fabricationand battery assembly involve substantial hand labor, partly due to thefrangible nature of MgO separators. The wafer pressing operation hasreceived some automation, but battery assembly relies on hand stackingof components.

Current thermal battery manufacturing employs uniaxial powder pressingtechnology to form active cell components. The thickness, diameter, andoverall geometry (parts are typically cylindrical) of the wafers arelimited by the uniaxial powder pressing process. The thicknessobtainable for uniaxially pressed wafers for thermal batteries rangesfrom approximately 1 mm to about 10 mm. Production of thinner or thickerparts is notably more difficult, and commonly results in low yields, andtherefore, higher costs. Thinner wafers require precise, even dieloading, while thicker wafers require the use of organic binders todistribute the applied pressure evenly. Similarly, large diameter wafersare difficult to uniaxially press due to increasingly larger processingequipment required to provide the necessary mechanical loads to form thewafers—typically greater than about 10,000 pounds-per-square inch (psi).These limitations preclude many advanced battery designs.

For electrode pellet manufacture, a high tonnage press typically isrequired to achieve 50 volume % active electrode material loading. Aportion of the electrolyte salt generally is combined with the electrodematerial to aid in the formation of suitable cold-pressed pellets. Themetal sulfide electrode material, FeS₂, is a very hard material and doesnot compact well on its own. Typically, the pressed electrode uses FeS₂coated with electrolyte salt to facilitate the powder compaction. Theresulting cold-pressed pellet generally comprises about 50 volume %FeS₂, about 30 volume % electrolyte salt, and a void volume of about 20volume %. An unpressed powder layer would typically have a void volumeof about 50 volume %. To achieve the desired 50 volume % active materialloading, the high tonnage press must displace about 30% of the voidvolume that is needed for the electrolyte salt. This is crucial, in thatunpressed electrodes with a 20-30 volume % loading of electrolyteexhibit poor performance (e.g., low energy density and low poweroutput).

The separator material used in previous molten salt thermal batteries ispressed from a high-surface area MgO powder. MAGLITE® S or MAGLITE® D(Calgon), and more recently MARINCO® OL (Marine Magnesium Company)magnesium oxide mixed with electrolyte salt, have been the materials ofchoice for pressed powder separators. Alternative materials have beeninvestigated, but only the pressed-powder MgO/salt separator has foundcommercial application.

SUMMARY OF THE INVENTION

The present invention provides a flexible, porous ceramic composite filmsuitable for use in a thermal battery, in which the film comprises 50%to 95% by weight of electrically non-conductive ceramic fiberscomprising a coating of magnesium oxide on the surface of the fibers inan amount in the range of 5% to 50% by weight. The ceramic fibers cancomprise Al₂O₃, AlSiO₂, BN, AlN, or a mixture of two or more of theforegoing, preferably Al₂O₃, AlSiO₂, or a combination thereof (e.g., 50%to 95% by weight Al₂O₃ and 5% to 50% by weight AlSiO₂). In aparticularly preferred film, the ceramic fibers comprise 56% by weightAl₂O₃ and 19% by weight AlSiO₂ and the magnesium oxide coating ispresent at 25% by weight. The magnesium oxide coating interconnects theceramic fibers providing a porous network of magnesium oxide-coatedfibers having a porosity of not less than 50% by volume (preferably 70to 95%). Preferably, the ceramic fibers are up to 10 microns in diameterand 1 mm in length. In some preferred embodiments the film has athickness of less than 12 mils (0.3 mm), e.g., 5 to 10 mils (about 0.13to 0.26 mm). Preferably, the film includes pores up to 5 microns inaverage diameter. Advantageously, the porous ceramic composite (PCC)films of the present invention are manufactured without the use of ahigh-pressure hydraulic press, and can be prepared in precursor sheetsthat can be readily cut to any desired shape or size up to 250 mmdiameter. The PCC films also exhibit surprising flexibility and handlingstrength.

In another aspect, the present invention provides a flexible compositefilm suitable for use in a thermal battery comprising 50% to 95% byweight of electrically non-conductive ceramic fibers coated withmagnesium oxide (e.g., in an amount in the range of 5% to 50% byweight), wherein the magnesium oxide coating interconnects the ceramicfibers providing a porous network of magnesium oxide-coated fibershaving a porosity of not less than 50% by volume, and the pores of thenetwork contain a solid electrolyte salt (e.g., an alkali metal halide,an alkali metal nitrate, an alkali metal nitrite, and the like) in anamount of up to 95% by volume based on pore volume of the network. Theceramic fibers can be Al₂O₃, AlSiO₂, BN, AlN, or a mixture of two ormore of the foregoing (preferably comprise Al₂O₃, AlSiO₂, or acombination thereof). The electrolyte salt preferably comprises alithium halide salt (e.g., a mixture of LiCl, LiBr, and KBr). As usedherein and in the appended claims, the term “solid electrolyte salt” andgrammatical variations thereof refers to an electrolyte salt that issolid at room temperature and molten at thermal battery operatingtemperatures.

In another aspect, the present invention provides a laminated electrodeand porous separator film combination suitable for use in thermalbatteries. The combination includes a solid electrolyte salt within aPCC film of the invention as a separator, and layer of powdered cathodematerial adhering to a surface of the PCC film with additionalelectrolyte therebetween to bind the cathode and the separator. The PCCfilm comprises 50% to 95% by weight of electrically non-conductiveceramic fibers having a coating of magnesium oxide on the surface of thefibers in an amount in the range of 5% to 50% by weight. The ceramicfibers can be Al₂O₃, AlSiO₂, BN, AlN, or a mixture of two or more of theforegoing. As in the other aspects of the invention, the magnesium oxidecoating interconnects the ceramic fibers providing a porous network ofmagnesium oxide-coated fibers having a porosity of not less than 50% byvolume (typically 70 to 95 volume %). In this aspect, the pores of thenetwork contain a solid electrolyte salt, such as an alkali metalhalide, nitrate or nitrite salt, in an amount of up to 95% by volumebased on the pore volume of the network. The cathode material caninclude one or more compound of Fe, Co, Cu, or Ni. Preferably, theseparator film contains fibers of Al₂O₃, is less than 12 mils (0.3 mm)thick, and includes an electrolyte containing LiCl present in an amountin the range of 80% to 95% by volume of the porous film.

Another aspect of the present invention is a thermal battery cellcomprising a lithium-containing anode material and a powdered cathodematerial separated by a flexible, porous composite film of the presentinvention, as described herein. Preferably, the battery comprises aplurality of the cells connected in series or parallel.

In yet another aspect, the present invention provides a method of makingflexible, porous ceramic composite film, comprising the steps of (a)depositing a mat of ceramic fibers onto a substrate from a suspension ofthe ceramic fibers; (b) introducing a soluble magnesium salt into themat of ceramic fibers; (c) drying the mat and magnesium salt to form adried, paper-like film; (d) and heating the dried film, preferably in anoxygen-containing atmosphere (e.g. in air), at a temperature and for atime sufficient to convert the magnesium salt to a coating of magnesiumoxide on the fibers. The MgO interconnects the ceramic fibers to form aporous network having a porosity not less than 50% by volume. The PCCfilms are flexible and have surprising and exceptional handling strengthsuitable for thermal battery use, at thicknesses in the range of about 3to 12 mils (0.077 to 0.3 mm), which is significantly thinner than thepractical limit for MgO pressed-powder films typically used as thermalbattery separator components. The mat of fibers can be deposited onto amesh screen or can be cast onto a substrate, such as a polyethyleneterephthalate (PET) film substrate.

In one aspect, this invention relates to the design and manufacture ofthermal batteries using a thin PCC film as a substitute to theconventional pressed MgO powder separator, which can be produced at athickness of about 3 to 12 mils while still maintaining a surprisinglyhigh degree of strength and flexibility. The thinner PCC film results ina significantly higher power (higher voltage and current) when used as aseparator in a thermal battery, allowing a greater portion of thebattery height to be utilized for increasing the number of cells (andthus voltage) in a battery of a given size. In addition, the thinnerseparator results in less battery of the battery volume and thermal massbeing inactive (i.e., non-electrolyte or electrode material), resultingin significantly higher battery energy per unit volume. Pulse power mayincrease by 50% using the PCC films of the invention as the separatorcomponent. Higher power density is a dominant theme in thermal batterydevelopment today. In one application, the PCC film may increase batterypulse power from 5.5 kW to about 8 kW.

The PCC films of the invention, when used as a separator for a thermalbattery, have unique chemical stability to Li activity and wetability tomolten halide electrolytes. In addition, the PCC films have excellenthandling characteristics (flexure strength) and durability in moltensalt. Thin PCC films withstands distortion from the volume changesduring cell discharge much better than conventional pressed-powder MgOseparators. Additionally, PCC films provide a foundation for a new,more-economic method of manufacturing thermal batteries. The structuralstability of PCC films in molten salt permit the electrode material tobe applied to the film in a continuous process, producing a laminate ofelectrode and separator. The ability to utilize larger diameterseparators, due to the flexibility and strength of the PCC films,enables new designs for high power thermal batteries. The ability of PCCfilms of the invention to increase battery power and energy density hasbeen demonstrated.

In some preferred embodiments, the ceramic fibers are present in the PCCfilm at a concentration in the range of about 70% to about 90% byweight, with the MgO coating being present in the range of about 10% toabout 30% by weight.

Advantageously, the PCC films of the present invention allow for animproved, continuous manufacturing method for producing thermal batterycells in which a handleable electrode of 40-60% active material isproduced without the use of a high tonnage hydraulic press. A thermalprocess integrates a PCC film separator with a bed of electrode materialparticles (e.g., iron sulfide) to produce a separator/electrodecombination. The PCC film separator enables molten electrolyte toinfiltrate the film and the metal sulfide electrode particle-bed toform, upon cooling, a unitary laminated structure having an electrodelayer and a separator layer bound together with the electrolyte andaffording a desirable electrolyte density of 50 volume % in theelectrode layer. The structural integrity of the PCC film providessupport for the structure, and as the electrolyte salt melts, theporosity of the PCC film regulates the flow of the salt over to theelectrode powder bed to keep the electrode material from fluidizing,which is undesirable.

The PCC films of the invention have exceptionally high porosity(typically 70 to 90 vol. %) compared to conventional MgO-type separatormaterials commonly used in thermal batteries, yet surprisingly, evenfilms as thin as 3 mils in thickness retain paper-like structuralintegrity and flexibility. These superior handling characteristics makethe PCC films of the invention particularly useful in thermal batteryapplications where very thin films are desirable, but expensive due tothe typically friable nature of conventional separators. In addition,the PCC films of the invention can be utilized in other applicationswhere porous films are useful, such as in liquid filtration or as aporous support for various chemical processes, for example as a catalystsupport.

Additional advantages, objects and novel feature of the invention willbecome apparent to those skilled in the art upon examination of thefollowing and by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts in cross-section, the process for making a laminate of aporous ceramic composite (PCC) film along with a pressed-powderFeS₂/electrolyte electrode.

FIG. 2 depicts the conveyor belt (a) process for making a laminatecross-section b-f of a PCC film along with a metal-sulfide,particle-bed, which after electrolyte addition becomes an electrode.

FIG. 3 is a graph of the modulus of rupture in pounds-per-square inch(psi) for a PCC film/electrolyte compared to a standard MgO/electrolytepressed-pellet-type material.

FIG. 4 is a cell voltage/time plot of a Li—Si alloy/CoS₂ cell includinga PCC film separator with 10 sec pulses at 0.9 A/cm².

FIG. 5 is a graph comparing the final voltage of 10 second pulses at 0.9A/cm² current density for the direct substitution of a 10 mil PCC filmfor a 10 mil pressed-powder MgO separator in a thermal battery.

FIG. 6 is a graph illustrating the low impedance of Li—Si alloy/CoS₂cell including a 10 mil PCC film separator, based on 10-sec pulses of0.9 A/cm².

FIG. 7 is a cell voltage/time plot of a Li—Si alloy/CoS₂ cell includinga PCC film separator compared to a cell with a MgO separator.

FIG. 8 shows a scanning electron microscopic (SEM) image of a PCC filmof the invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to ceramic articles and methods of making same,which are electric insulators useful for a variety of purposes. However,for illustration purposes, the invention will be described in connectionwith its use as a separator in a thermal battery. A preferred PCC filmseparator consists of about 10/90 to about 30/70 weight ratio of ceramicfiber/MgO coating. Fiber diameter preferably is about 10 microns andfiber length preferably is about 1000 microns. A PCC film of theinvention comprises or consists of resilient ceramic fibers coated withmagnesium oxide and formed into a film of about 3 to 12 mil thickness.The PCC film can accommodate a loading of at least 85 volume % ofelectrolyte to impart high ionic conductivity. A particularly preferredseparator PCC film composition is 56% Al₂O₃ fiber, 19% AlSiO₂ fiber, 25%MgO coating, by weight. An example of useful fiber blend is 75/25 weight% Al₂O₃/AlSiO₂. Fiber sources include ALTRA® Al₂O₃ from Rath(Wilmington, Del.) and FIBERFRAX® AlSiO₂ from Carborundum.

The MgO coating material is formed on the ceramic fibers in situ bythermal decomposition of the soluble magnesium salt (e.g., a magnesiumsalt of a carboxylic acid, such as magnesium acetate; magnesiumcarbonate; and the like). The magnesium salt coats the fibers and thenis converted to magnesium oxide, which also binds the ceramic fiberstogether for flexibility and strength, while leaving an open porousstructure with greater than 50 volume % of void space in the film. ThePCC films of the invention are particularly useful as separators inthermal batteries, since the films possess durability and flexibilityfar beyond that of conventional MgO pressed-powder separators.

Preferably, the MgO coating contributes about 30% by weight of the film.Examples of additional useful fibers include boron nitride (BN) andaluminum nitride (AlN). SiO₂ or ceramic/glasses with higher levels ofSiO₂ have been found to be of insufficient chemical stability for thehigh Li-activity Li-alloy electrodes that are generally used in thermalbatteries. The use of a precursor that decomposes into a MgO coating inthe methods of the present invention further enhances the chemicalstability of the resulting film e.g., when used as a separator in athermal battery. Magnesium acetate is a particularly useful MgOprecursor, although any other thermally decomposable magnesium salt(e.g., other magnesium carboxylic acid salts, magnesium carbonate, andthe like), which are well known in the art will suffice.

In a preferred method embodiment, the ceramic fibers are “dropped” ontoa fine mesh (e.g., a polyester mesh) at a laydown of about 3 mg/cm² toprovide an approximately 130 micron thick layer, a series drying andinfiltration steps coat the fibers with and connect the interstices ofthe fiber mat with magnesium oxide. As used herein, the term “dropping”and grammatical variations thereof, refers to a technique used inpapermaking in which fibers are collected on a fine screen by filtrationof an aqueous suspension of fiber. A solution of a soluble magnesiumsalt (e.g., magnesium acetate) is applied to the fiber mat (e.g., as a0.6 g/cm³ aqueous solution). Magnesium carbonate and magnesium hydroxideare two other non-limiting examples of soluble magnesium salts thatcould be substituted for magnesium acetate. Optionally, a liquid dryingagent, such as isopropyl alcohol or the like, can be applied to the matafter the magnesium salt to aid in wetting ceramic fibers and to enhancethe drying. Drying preferably is performed in a flowing stream of air atabout 75 to about 100° C. After drying for about 2 hours, the resultingceramic fiber “paper” can be peeled from the fine mesh. A combustiblecarrier can be used to facilitate the peeling step, if desired. Theceramic fiber paper is sufficiently rigid for good cutting, e.g., with adie punch, yet it can be handled and has sufficient flexure strengththat it doesn't easily crack and break apart.

Ceramic fiber paper as thin as 100 microns can be handled in sheets aslarge as 250 mm diameter without significant breakage. The process iscompleted by heating the ceramic fiber paper to a temperature sufficientto convert the magnesium salt to MgO (e.g., about 600 to 650° C.). Anexemplary method of preparing a PCC film of the invention has thefollowing steps: (a) blending and dispersing fibers with water (e.g., inan impeller); (b) introducing the fiber suspension onto a fine mesh(e.g., polyester vale); (c) removing water (e.g., by filtration or bysimple drainage); (d) drip drying the resulting fiber mat; (e)introducing an aqueous magnesium salt solution into the fiber mat toinfiltrate the fiber mat with magnesium salt; (f) drying the infiltratedmat in flowing hot air; (g) preferably repeating the infiltration steps(e) and (f) at least once; (h) peeling the resulting fiber paper fromthe fine mesh; (i) cutting the paper to a desired size; and (j) heatingthe cut paper at about 600-650° C. in air for about 3 to 6 hours toconvert the magnesium salt to MgO.

Non-limiting examples of other methods useful for forming ceramic fibersinto a mat for use in producing PCC films of this invention include:

1. pulling fibers from a fluidized bath containing ceramic fibers onto abelt (e.g. fine mesh screen or combustible carrier) e.g., using a vacuumroller;

2. spraying an aqueous suspension of ceramic fibers onto a belt (e.g.fine mesh screen or combustible carrier);

3. slip casting a suspension of ceramic fibers in gelatinous medium ontoa belt or other substrate (e.g. fine mesh screen, a film, or combustiblecarrier); and

4. blowing an air dispersion of ceramic fibers onto a belt (e.g. finemesh screen or combustible screen).

Slip casting a slurry or suspension onto a substrate such as a PET filmis a preferred method for preparing a fiber mat.

The present invention provides flexible PCC films at about 3 to 12 milthickness, which is a significant improvement over the present thinnestlimits of 25 mil for cell pressed wafers at 10 mm diameter or larger.Because they are also supplied in the form of flexible sheets, theflexible, porous ceramic composite films of the invention offer costsaving options for thermal battery manufacture. The cost of conventionalthin-cell thermal batteries is inflated by the poor handlingcharacteristics of the wafer-thin components. A thirty percentparts-loss rate is presently typical. Even at the conventionalthicknesses, the thermal batteries can benefit from using the PCC filmsof the present invention as the separator component. The fragility ofconventional wafer pellets has required expensive hand assembly. Thedurability of the films of this invention (e.g., the bendable nature ofthe films and the ability to pass a “drop” test) permits automated,faster assembly. Reduction of human error from the assembly processimproves quality control, thereby further increasing the profitabilityfor thin-cell thermal batteries. In addition, the PCC films of theinvention can be readily wetted with molten electrolyte salts (e.g.,alkali metal halide), exhibit good bending strength, flexibility, smallpore-size, low density, and tortuosity, which are highly desirablefeatures or properties for an improved separator in thin-cell,high-power thermal batteries.

Cell Fabrication with Porous Ceramic Fiber Films.

The structural stability of PCC films in the presence of moltenelectrolyte salt provides the basis for a significantly improved methodfor thermal cell manufacture. In this method of thermal batterymanufacture, the PCC film acts as a buffer to regulate the amount ofmolten electrolyte that is applied to a metal-sulfide, particle-bedelectrode. Use of too much electrolyte undesirably tends to fluidize theparticle bed, thus destroying the packing-density and the physicaldimensioning of the wafer-thin electrode. The PCC film of the inventioncan be used in at least three variations of thermal battery cellmanufacture, which results provide a PCC film/metal-sulfide electrodelaminate. The laminate significantly enhances the handling strength of aseparator/electrode combination, and also helps to ensure propercomponents mating and flatness for stacking the cells. Typically, theform of the metal sulfide electrode will dictate the fabricationprocedure to be used.

A conventional metal-sulfide electrode typically is a pressed-powder bedof metal sulfides and an electrolyte salt. The process enables theelectrolyte to infiltrate the metal sulfide particle-bed matrices andretain the initial desired particle-bed density of 50 volume %. In theprior art, production of metal sulfide electrode pellets withparticle-bed density that approached 50 volume % required high-tonnagehydraulic presses. The salt component of the pellet was compactedbetween the metal sulfide particles by the elimination of void volume.The present invention infiltrates molten salt in a controlled fashioninto a particle-bed, and can achieve the same particle-bed density, 50volume %, without the high-tonnage hydraulic presses. Alternatively, asimple metal sulfide particle bed contained within a cup, or a typecastlayer of metal sulfide particles in an expendable binder matrix can beused to form the electrode, in which case an electrolyte is not neededto form the laminate.

The cross-sectional views in FIGS. 1( a)-(d) depict the process formaking a laminate of a PCC film and a pressed-powder FeS₂ electrodebound together by an electrolyte salt. The laminated component 30enables a thin, large-diameter separator and cathode combination to beassembled into a thermal battery. In FIG. 1 a, the first step involvesplacing an electrolyte powder 10 onto a conveyor that passes through atunnel furnace, in an amount necessary to infiltrate the a PCC film. Theelectrolyte powder can be dispensed by a shoe (a powder filled hopper)traveling over a cavity (not shown), or as a die-punched piece oftapecasted electrolyte powder. A PCC film 12 is placed onto electrolytepowder 10. In turn, a pressed-powder FeS₂/electrolyte electrode 16 isplaced onto the PCC film 12. The stacked components then travel throughthe tunnel furnace at about 550° C. for about 2 minutes. As shown inFIG. 1 b, the electrolyte powder 10 melts and infiltrates into PCC film12 to form a PCC film/electrolyte salt combination 20. Thepressed-powder FeS₂/electrolyte electrode 16 remains on top. The stackedcomponents then travel out of the tunnel furnace, and onto a chillingblock 42 (e.g., a copper block) as shown in FIG. 1 c. After cooling toroom temperature on block 42 over about 2 minutes, the resultinglaminated PCC film/cathode 30 emerges, as shown in FIG. 1 d. Thesolidified electrolyte salt binds the PCC film and electrode layers forsuperior handling. The laminated PCC film/cathode 30 is immediatelyavailable for assembly of a thermal battery (e.g., by stacking with ananode, a heat pellet and a current-collector sheet).

The cross-sectional view of FIGS. 2 a-f depict the process for making alaminate of a PCC film along with a FeS₂ powder bed (i.e., eliminatingthe use of hydraulic pressing for producing the electrode). Thelaminated component 30 enables thin, large diameter separator andcathode combinations to be assembled into a thermal battery. As shown inFIG. 2 a, the process uses a conveyor belt 50 that consists of plateswith shallow cups. In FIG. 2 b, the first step involves placing anamount of metal sulfide powder 8 onto a conveyor that moves through atunnel furnace. The powder can be dispensed by a shoe (a powder filledhopper) traveling over a cavity (not shown) or as a die punched piece oftapecasted electrode powder. In FIG. 2 c, a PCC film 12 is placed ontothe electrode powder 8. In turn, FIG. 2 d illustrates placing anelectrolyte powder 10 onto the PCC film 12. The amount of electrolytepowder 10 placed onto the PCC film 12 is just enough to infiltrate boththe PCC film 12 and the FeS₂ powder bed 8. Again, the electrolyte powdercan be dispensed by a shoe (a powder filled hopper) traveling over acavity (not shown) or as a die punched piece of tapecasted electrolytepowder. The stacked components are then conveyed through the tunnelfurnace at about 550° C. for about 2 minutes. Batch processing in avacuum oven (e.g., at about 550° C.) can also be used to melt theelectrolyte. As shown in FIG. 2 e, the electrolyte powder 10 melts andinfiltrates into PCC film (forming electrolyte filled film 20) and theFeS₂ cathode 16. The resulting combination 30 of PCC film and electrode,bound by electrolyte, is then conveyed out of the tunnel furnace, whereafter cooling to room temperature over about 2 minutes the laminated PCCfilm/cathode combination 30 is ejected (see FIG. 2 f) by part-ejector 40(a push plate) that is at the bottom of each cup on the conveyor.

The solidified electrolyte salt along with the high modulus of rupture(MOR) of the PCC film of the invention, unitizes the separator (PCCfilm) and electrode layers for superior handling. The laminated PCCfilm/cathode combination 30 is immediately available for assembly of athermal battery, as described above.

Examples of preferred fibers for use in the PCC films of the inventiongenerally have a diameter of about 10 μm (micrometers) and a length ofabout 1 mm. Ceramic fibers are generally manufactured to a nominal fiberdiameter of between 3-4 μm, although a typical range of actual diametersis 0.2-8.0 μm.

The following examples are provided to illustrate certain aspects of thepresent invention and are not to be interpreted as limiting theinvention in any way.

EXAMPLE 1 Production of a PCC Film

A PCC film is prepared by using a blender to suspend 3.0 grams ofceramic fiber (composition of 75/25 weight % Al₂O₃/AlSiO₂ comprisingSAFFIL® Al₂O₃ fibers from ICI, and FIBERFRAX® AlSiO₂ from Carborundum)in 0.5 liters water. The fiber then is dispersed in 8 liters of water ina papermaking machine and dropped onto a 250 mm diameter fine polyestermesh at a 6 mg/cm² loading to provide a 250 micron thick fiber matlayer, which is then drip dried. The mat then is infiltrated at withmagnesium acetate (applied as a 0.6 g/mL aqueous solution containing 5volume % isopropanol) and dried. The infiltration and dying is repeatedat least once to provide a ceramic fiber paper. Addition of isopropanolaids in wetting the ceramic fibers and enhances the drying rate. Dryingis done in flowing air at about 75-100° C. After drying about 2 hours,the resulting ceramic fiber paper can be peeled from the fine polyestermesh. The pieces of ceramic fiber paper are cut to a desired size usingan EXACTO® knife and a precision form, such as a 2.05 inch diameterform. The cut paper next is processed at 600-650° C. in air for 3-6hours to convert the magnesium acetate to MgO, forming a porous ceramiccomposite film of the invention. The film has a 70/30 weight ratio ofAl₂O₃ fiber-to-AlSiO₂ fiber.

Electrolyte is infiltrated into the PCC film by placing a weighed amountof electrolyte powder onto the PCC film, and placing it onto a 500° C.hot plate just long enough to melt the electrolyte. Theelectrolyte-infiltrated piece is then placed onto chill-block (e.g. a Moplate) and cooled under a weight to solidify the electrolyte. Thiselectrolyte-filled PCC film can be used as a separator in a thermalbattery cell by stacking pressed-pellets of a Li—Si alloy/electrolytewafer and a CoS₂/electrolyte wafer on either side of the PCC film toform a test cell.

An important specification related to the handling strength ofseparators for thermal batteries is the modulus of rupture (MOR) orbending strength before breaking. As the separator is thinned, itbecomes easier to break. A three-point break apparatus is used toevaluate MOR. The MOR is determined by incrementally-loading thethree-point fixture (usually three rods in parallel) until the specimensnaps. Separator MOR values are determined from a group of repeatedtests. The MOR is normalized for varying cross-section.

The PCC film surprisingly has an MOR of 2,000 up to 4,500 compared toonly about 100 for the conventional pressed-powder MgO/electrolyte saltpellet (i.e., the PCC film has at least 20 times greater than that ofthe MgO powder separator), as illustrated in FIG. 3, and this is what ismeant by use of the term “flexible” regarding the PCC films of thepresent invention. Since the bending moment increases for a largerdiameter separator, the MOR becomes more critical for larger diametercells. The PCC films of the invention surprisingly have the samehandling strength as a standard pressed-powder MgO separator at only 5%the thickness of the MgO separator. It is therefore understandable thatthe CFS at 50% thickness of the MgO separator thickness has superiorhandling strength. Thus, the PCC film, at 50% thickness of a standardMgO separator, exhibits far superior handling strength, and alsofulfills the targeted power density of the emerging thermal batterymarket.

Additionally, the PCC films of this invention, when used as separatorsin a thermal battery cell, have the chemical and physical propertiesnecessary to meet the goal of high current density at high power forfuture thermal battery applications (i.e., 85-95% open volume for highelectrolyte content, and the chemical stability to provide resistance toLi corrosion). Unlike pressed-powder MgO separators, full-size 3.66 inchdiameter PCC film separators pass the “drop test”, and displaysurprising physical flexibility even after electrolyte filling. The PCCfilms of the invention can reduce cell thickness and weight, allowingfor approximately 15% more cells per unit of height for a thermalbattery utilizing PCC film separators compared to a battery usingpressed-powder MgO separators.

EXAMPLE 2 PCC Film Laminated with Pressed Electrode Pellet

A PCC film is prepared by using a blender to suspend 1.5 grams ofceramic fiber (composition of 75/25 weight % Al₂O₃/AlSiO₂ comprisingALTRA® Al₂O₃ fibers from Rath, Wilmington, Del., and Z-90 SAZ® P-15AlSiO₂ from K Industries, Livonia, Mich.) in 0.5 liters water. The fiberthen is dispersed in 8 liters of water in a papermaking machine anddropped onto a 250 mm diameter fine polyester mesh at a 6 mg/cm² loadingto provide a 250 micron thick fiber mat layer, which is then drip dried.The mat then is infiltrated with magnesium acetate (applied as a 0.6g/mL aqueous solution containing 5 volume % isopropanol) and dried. Theinfiltration and dying is repeated at least once to provide a ceramicfiber paper. Addition of 5 volume % isopropanol aids in wetting theceramic fibers and enhances the drying rate. Drying is done in flowingair at about 75-100° C. After drying about 2 hours, the resultingceramic fiber paper can be peeled from the fine polyester mesh. Thepieces of ceramic fiber paper are cut to a desired size using an EXACTO®knife and a precision form, such as a 2.05 inch diameter form. The cutpaper next is processed at 600-650° C. in air for 3-6 hours to convertthe magnesium acetate to MgO, forming a porous ceramic composite film ofthe invention having a 70/30 weight ratio of ceramic fiber to MgO. ThePCC film has a thickness of less than about 12 mils (generally 5-10,preferably 4-6 mils), and has far superior handling characteristicscompared to a conventional MgO separator. A cut piece of the PCC film isthen stacked with a CoS₂/electrolyte pressed-pellet electrode, as shownin FIG. 1.

Electrolyte is infiltrated into the PCC film by placing 1.28 grams ofelectrolyte powder (an equimolar (i.e., 1:1:1 molar) blend of LiCl,LiBr, and KBr) onto the PCC film, and placing PCC film/electrode stackonto a 500° C. hot plate just long enough to melt the electrolyte intothe PCC film. The electrolyte-infiltrated PCC film/electrode combinationis then placed onto chill-block (e.g. a Mo plate) and cooled under aweight to solidify the electrolyte, laminate the PCC film to theelectrode via the solidified electrolyte. The resultingelectrolyte-to-separator weight ratio is about 87:13. Thiselectrolyte-filled PCC film/CoS₂ electrode is then stacked with a Li—Sialloy/electrolyte pressed pellet wafer to form a test cell.

The test cell was evaluated under static thermal conditions with 10second pulses using 0.9 A/cm² current density at 500° C., see FIG. 4.Outstanding performance of greater than 1.6 volts for the pulse voltagefor the first 75% of cell capacity showed that PCC films of theinvention (at less than 12 mils thickness) can meet or exceed theperformance of the pressed MgO powder separators of substantiallygreater thickness.

EXAMPLE 3 PCC Film Laminated with Electrode Particle Bed

A cut 250 micron thick PCC film as produced in Example 1 is positionedonto a cavity that has been filled with FeS₂ particles, as shown in FIG.2. The cavity is coated with BN to eliminate sticking to the cup. Inthis procedure, 2.3 g of the LiCl—LiBr—KBr electrolyte mixture used inExample 2 was placed onto the PCC film—this amount of electrolyte issufficient to infiltrate both the PCC film and electrode FeS2 electrodepowder. The arrangement of materials is then passed through a 550° C.tunnel furnace. After the electrolyte melts and infiltrates the twocomponent layers, the electrolyte-to-separator weight ratio is 87:13 andthe electrolyte-to-FeS₂ cathode weight ratio is 25:75. Theelectrolyte-infiltrated PCC film becomes laminated to theFeS₂/electrolyte pellet via the infiltrated electrolyte salt.

EXAMPLE 4 PCC Film Laminated with a Tapecast Electrode Particle Bed

A tapecast electrode is comprised of particles held together in 2-5volume % of a polymer matrix, which is decomposed and removed duringthermal processing of the electrode. A PCC film comprising a 80:20weight ratio of Al₂O₃-to-AlSiO₂ fiber, as formed in Example 2, ispositioned onto a tapecast piece of cathode material (i.e., a powder bedof 50 volume % FeS₂ and CuFeS₂ particles in a 7:3 molar ratio,respectively) of the same size and shape and the stack is placed onto aconveyor belt that runs through a tunnel furnace. The conveyor belt iscoated with BN to eliminate sticking. About 2.3 g of the LiCl—LiBr—KBrelectrolyte material used in Example 2 is placed onto the PCC film,which then is passed through the tunnel furnace at a 550° C. After theelectrolyte melts and infiltrates the two component layers, theelectrolyte-to-separator weight ratio is 89:11 and theelectrolyte-to-cathode weight ratio is 22:78. Theelectrolyte-infiltrated PCC film is laminated to the cathode/electrolytepellet via the infiltrating electrolyte. The laminated PCC film/cathodecomponent which possesses superior handling strength compared toconventional MgO-based separator material, is then stacked with Li—Sialloy/electrolyte pellet to form a thermal battery cell.

EXAMPLE 5 Third-Party Test of PCC Film

A 2.5 inch diameter, 10 mil thick PCC film of the invention, as preparedin Example 2 was tested at a third-party test facility in Li—Sialloy/CoS₂ cells, compared to cells containing a conventional 15 milthick MgO separator. The tests used 29 Amp pulses for 10 seconds everyminute (see FIG. 7, in which voltage trace 1 is for the conventionalMgO-based cell, and trace 2 is for the PCC-based cell of the invention).In spite of the lower level of electrolyte present in the PCC film-basedcell, the PCC cell surprisingly exhibited increased power, with adesirable 33% decrease in cell impedance for the first half of thecell's upper plateau capacity. In addition, the cell capacitysignificantly increased due to the use of the PCC film as a separator,as evidenced by the two additional pulses that were obtained. The lowerseparator weight in the PCC-based cell also allowed for the use of alower weight heat pellet.

In summary, the PCC film materials of the present invention have thefollowing beneficial properties: MOR of Salt-Loaded Parts (>2000 psi);Average bulk density without electrolytes (0.3 g/cm³); Typical OpenVolume (90-95%); Thickness range (0.003-0.25 inches); Tensile Strength(>350 g/in); and Maximum Use Temperature (about 1200° C.). Table 1illustrates typical values for important characteristics of a PCC filmof the invention compared to those of a pressed MgO powder for use as aseparator in a thermal battery cell.

TABLE 1 Separator Comparison MgO Powder Characteristics Pellet CeramicFiber Film Separator Thickness 10-25 mil 5-10 mil Cost, % of Total ~3%~10% of Thin Cell Materials Size Limitation 3.5 in Up to 10 inchDiameter achieved Diameter Handling Brittle Flexible Electrolyte Content70 vol. % 85 vol. %

The PCC films of the present invention are flexible and highly porous,while exhibiting an unexpectedly high MOR of 2000 to 4500 psi, makingthese materials an excellent replacement for conventional MgO separatorsin thermal battery applications, even at thicknesses of 5 to 10 mils.FIG. 8 shows a scanning electron microscopic (SEM) image of a PCC filmof the invention, which shows the ceramic fibers coated with MgO and theMgO coating connecting the fibers together in a network. The image inFIG. 8 also clearly illustrates the open pore structure of the PCC film,which is formed by the interstices between the interconnected fibers.

All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and specifically indicatedto be incorporated by reference and were set forth in its entiretyherein.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. The term consisting of is tobe construed as limiting the scope to specified materials or steps. Theterm consisting essentially of is to be construed as limiting the scopeto specified materials or steps and those that do not affect the basicand novel characteristics of the claimed invention. Recitation of rangesof values herein are merely intended to serve as a shorthand method ofreferring individually to each separate value falling within the range,unless otherwise indicated herein, and each separate value isincorporated into the specification as if it were individually recitedherein. Recitation of numbers values are to be interpreted as includingknown suitable margins of measurement error consistent with thetechnique exemplified as being used to determine the value. All methodsdescribed herein can be performed in any suitable order unless otherwiseindicated herein or otherwise clearly contradicted by context. The useof any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

Preferred embodiments of this invention are described herein, includingthe best mode known to the inventors for carrying out the invention.Variations of those preferred embodiments may become apparent to thoseof ordinary skill in the art upon reading the foregoing description. Theinventors expect skilled artisans to employ such variations asappropriate, and the inventors intend for the invention to be practicedotherwise than as specifically described herein. Accordingly, thisinvention includes all modifications and equivalents of the subjectmatter recited in the claims appended hereto as permitted by applicablelaw. Moreover, any combination of the above-described elements in allpossible variations thereof is encompassed by the invention unlessotherwise indicated herein or otherwise clearly contradicted by context.

1. A flexible, porous ceramic composite film comprising 50% to 95% byweight of electrically non-conductive ceramic fibers, a coating ofmagnesium oxide on the surface of the fibers in an amount in the rangeof 5% to 50% by weight of the film; wherein the ceramic fibers compriseAl₂O₃, AlSiO₂, or a mixture of the foregoing; and the magnesium oxidecoating interconnects and binds the ceramic fibers together, providing aporous network of magnesium oxide-coated fibers having a porosity of notless than 50% by volume.
 2. The film of claim 1, wherein the ceramicfibers are up to 10 microns in diameter and up to 1 mm in length.
 3. Thefilm of claim 1, wherein the ceramic fibers comprise 50% to 95% byweight Al₂O₃ and 5% to 50% by weight AlSiO₂.
 4. The film of claim 1,wherein the porous network has a porosity in the range of 70 to 95% byvolume.
 5. The film of claim 1, wherein the film has a thickness of lessthan 12 mils.
 6. The film of claim 1, wherein the film has a thicknessin the range of 5 to 10 mils.
 7. The film of claim 1, wherein the porousfilm includes pores up to 5 microns in average diameter.
 8. The film ofclaim 1, wherein the ceramic fibers comprise 56% by weight Al₂O₃ and 19%by weight AlSiO₂ and the magnesium oxide coating is present at 25% byweight.
 9. A flexible, porous ceramic composite film comprising 50% to95% by weight of electrically non-conductive ceramic fibers, a coatingof magnesium oxide on the surface of the fibers in an amount in therange of 5% to 50% by weight of the film; wherein the ceramic fiberscomprise Al₂O₃, AlSiO₂, or a mixture of the foregoing; and the magnesiumoxide coating interconnects and binds the ceramic fibers together,providing a porous network of magnesium oxide-coated fibers having aporosity of not less than 50% by volume; the pores of the networkcontaining a solid electrolyte salt in an amount of up to 95% by volumebased on pore volume of the network.
 10. The film of claim 9, whereinAl₂O₃ is present in the ceramic fibers and the film has a thickness lessthan 12 mils.
 11. The film of claim 9, wherein the electrolyte saltcomprises a lithium halide salt.
 12. The film of claim 9, wherein theelectrolyte salt comprises a mixture of LiCl, LiBr, and KBr.
 13. Thefilm of claim 9, wherein the ceramic fibers comprise 50% to 95% byweight Al₂O₃ and 5% to 50% by weight AlSiO₂.